Methods Of Control

A variety of control systems have been developed for use with mechanical adjustable-speed drives. For the majority of pump applications, speed is controlled manually through a lever, handwheel, or knob attachment. Remote semiautomatic and automatic control methods in mechanical, pneumatic, or electric forms are also being used.

For manual operation, vernier attachments are often useful to increase the accuracy of speed adjustment. Cams are occasionally employed, mounted externally or internally, to assure a prescribed pattern of output characteristics.

Remote control is usually obtained by means of a positioning motor, which is a fractional-power motor connected to the drive control shifting screw through reduction gearing. The output speed of the drive is then adjusted from a station at a remote location.

For semiautomatic or automatic operation, control systems usually consist of three elements: a sensing unit, a receiver, and a positioning actuator. The sensing unit detects changes in the process being controlled and transmits a signal to the receiver. At the receiver, the signal is analyzed, amplified, and transmitted to the positioning actuator, which adjusts the speed of the mechanical adjustable-speed drive accordingly.

If the process or load requirements can be adapted to produce a signal, there should be a suitable control system that can be used for speed adjustment. The only limitation is that the load requirements must follow a specific pattern of some type, regardless of whether the pattern is based on direct, inverse, or proportional relationships.

The pneumatic actuators used for speed adjustment are usually responsive to a 3 to 15-lb/in2 (20- to 100-kPa) air signal pressure. These pneumatic positioning devices are actually analog piloted servovalve positioning devices that can be used in a great variety of open- and closed-loop process control applications. They can be used to cause a mechanical adjustable-speed drive to either flow or maintain a given process signal from variables such as liquid level, pressure, flow rate, or any other measurable value. The adjustable-speed drive thus becomes the final control element in a closed-loop process control system. Figure 7 is a typical drive equipped with a pneumatic actuator for speed changing.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

Get My Free Ebook


Post a comment